Although canine kidneys can be stored successfully for up to 3 days by continuous pulsatile perfusion with cryo-precipitated plasma at 10 degrees C there is continued deterioration of renal reserve during this period of storage. Thus, dogs receiving kidneys stored for 24 hours will have lower serum creatinines in the postoperative period than kidneys stored for 72 hours. The exact mechanism of this deterioration is unknown and actually may be due to several different causes. Thus, substrate utilization may play a key factor. Our studies so far have shown -hydroxy-butyrate added can account for over 68 percent of the oxygen consumption of the tissues, but we are not able as yet to supply all metabolic fuel by the addition of a single substrate during hypothermic metabolism of the kidney cortex. Thus, endogenous substrate utilization continues and by itself could be the cause of many problems. Thus, exhaustion of endogenous substrate may play a role as oxidation of substrates such as lipids may cause direct harm by its effect on cellular membranes. We are planning to continue our investigations in the substrate utilization in both 38 degrees as well as 10 degrees C until we have found the mixture of substrates that will prevent utilization of endogenous substrates. Substrate utilization will be studied both aerobically as well as anaerobically. As anaerobic perfusion will prevent the obligatory activation of respiratory enzymes, this may be the preferred method for long-term storage. The search will continue, therefore, to attempt to provide the kidney cortex with substrate that can be utilized anaerobically. Finally, as one of the main problems in long-term storage may actually lie in hypothermia, perfusion at higher temperatures will be continued. This would prevent some of the physical changes which occur in the cell membranes of homeotherms, such as dog and man under conditions of hypothermia and also would eliminate the problems of cold sensitivity of certain essential enzyme systems. It still appears to be even more essential at the present time to obtain consistent long-term preservation (7 days or more) as evidence accumulating that the mixed lymphocyte culture reaction may indeed be the preferred method for donor/recipient matching, and that a breakthrough in clinical renal transplantation may not come until long- term preservation has been accomplished.